The present invention generally relates to semiconductor devices and more particularly to a laser amplifier supplied with a traveling optical beam and for amplifying the same by the stimulated emission.
In the long range optical telecommunication systems, it is necessary to provide optical repeaters in the optical fiber network for compensating the optical loss. Typically, the laser amplifiers are used for such optical repeaters. BY using the laser amplifiers, it is possible to amplify the optical signals on the optical fiber network without converting the same once to an electrical signal and further converting the electrical signal to the optical signal. Thereby, the construction of the optical telecommunication network is significantly simplified.
FIG.1 shows the construction of a typical conventional laser amplifier used for the optical repeaters.
Referring to FIG.1, the laser amplifier is formed on an n-type substrate 1 of InP. As illustrated, the substrate 1 is formed with a mesa structure 1a that extends like a stripe in the longitudinal direction.
On the upper major surface of the mesa la, there is provided an n-type waveguide layer 2 of In-GaAsP, and an undoped active layer 3 of InGaAsP is provided further on the layer 2. Further, a p-type clad layer 4 of InP is provided on the active layer 3. On the upper major surface of the clad layer 4, an upper electrode 5 is provided in ohmic contact therewith. On the lower major surface of the substrate 1, on the other hand, there is provided another ohmic electrode 6. Thereby, the waveguide layer 2, the active layer 3 and the clad layer 4 form a stripe-like region corresponding to the mesa 1a. Further, InP layers 7a and 7b are provided at both sides of the stripe-like region for confining the path of current injected from the electrode 5 and flowing to the electrode 6 through the structure of the laser diode.
The device of FIG.1 lacks the usual reflectors at both longitudinal ends. Thereby, the laser oscillation does not occur even when the electrodes 5 and 6 are biased to the extent that the laser diode oscillates when there are such reflectors. Upon incidence of an optical beam, the stimulated emission occurs in the device in response to the passage of the wavefront of the optical beam and the optical beam is amplified as is travels from one end to the other end of the device.
In such an operation of the optical amplification, it will be noted that a large gain is achieved by increasing the optical confinement factor .GAMMA. such that the optical beam is effectively confined in the active layer 3 as it travels through the device. On the other hand, when the optical confinement factor .GAMMA. is excessive, there is a tendency that the active layer 3 is saturated by the photons in the region close to the output end. When saturated, no or reduced stimulated emission occurs even when the optical beam passes through the active layer 3. Thereby, the gain of the optical amplification is saturated at the region close to the output end of the laser amplifier.
FIGS.2(A) and 2(B) are diagrams showing the saturation of optical amplification, wherein FIG.2(A) shows the vertical cross section of the diode of FIG.1 taken along the longitudinal direction while FIG.2(B) shows the optical intensity profile in the active layer 3. In FIG.2(A), the curve designated as 3a represents the optical intensity profile taken along the vertical cross section that is perpendicular to the traveling direction of the optical beam. It will be seen that there is a strong concentration of optical intensity in the active layer 3.
In this conventional example, it will be noted that the intensity of the optical beam increases with the traveling of the optical beam at the region close to the input end while the increase in the intensity becomes gentle and approaches to a level Pm at the output end. Once reached to the level Pm, the optical intensity and hence the optical output of the laser amplifier does not increase even when the longitudinal length is increased further.
FIGS.2(C) and 2(D) are diagrams corresponding to FIGS.2(A) and 2(B) for showing the optical amplification in a laser amplifier wherein the optical confinement factor .GAMMA. is reduced. As shown in FIG.2(C) by a curve 3b, the optical intensity distribution in the vertical cross section of the laser amplifier is more diffused as compared with the structure of FIG.2(A). In the illustrated example, such a reduction in the optical confinement factor .GAMMA. is achieved by reducing the thickness of the active layer 3 with respect to the waveguide layer 2.
In this example, the optical intensity in the active layer 3 increases generally linearly with the propagation of the optical beam from the input end to the output end. However, due to the reduced optical confinement in the active layer, the magnitude of the optical intensity in the active layer 3 is substantially smaller than the device of FIG.2(A).
In order to avoid the optical saturation and to obtain the linearly increasing optical power, Japanese Laid-open Patent Application 1-268084 proposes a structure wherein the lateral width of the active layer increases generally linearly from the input end to the output end. By constructing the active layer as such, the photon density in the active layer is maintained substantially constant at both the input end and the output end.
This conventional laser amplifier suffers from the problem that the quality of crystal forming the active layer tends to be degraded. More specifically, due to the existence of the side walls extending mutually with an angle, there is a tendency that defects such as dislocations tend to be formed in the active layer and such defects act as the recombination center for annihilating the carriers irrespective of the passage of the optical wavefront. Thereby, the efficiency of optical amplification and hence the optical gain of the laser diode is inevitably decreased.